Using the DNS in Visual Basic .NET

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Using the DNS
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At its core, the DNS is still a list of names and their IP addresses, but instead of storing all the information in one place, the DNS distributes it among servers all over the Internet. The DNS consists of a hierarchical namespace, a collection of name servers, and DNS clients called resolvers. Each name server is the authoritative source for a small part of the namespace. When DNS servers receive name resolution requests from
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Planning a Name Resolution Strategy
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resolvers, they check their own records for the IP address associated with the requested name. If the server does not have the information needed, it passes the request to other DNS servers, until it reaches the authoritative server for that name. That authoritative server is the ultimate source for information about that name, so the IP address it supplies is considered definitive. The authoritative server returns a reply containing the IP address to the requesting server, which in turn relays it back to the resolver, as shown in Figure 4-1.
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Request Reply Resolver DNS Server Authoritative DNS Server Request Reply
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Figure 4-1 DNS servers relay requests and replies to other DNS servers
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For the DNS to function in this manner, it was necessary to divide the namespace in a way that would distribute it among many servers. It was also necessary to devise a methodology that would enable a server to systematically locate the authoritative source for a particular name. To accomplish these goals, the developers of the DNS created the concept of the domain. A domain is an administrative entity that consists of a group of hosts (which are usually computers). When a DNS server is the authoritative source for a domain, it possesses information about the hosts in that domain, in the form of resource records. The most common resource record is the Host (A) resource record, which consists of the host name and its equivalent IP address.
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Off the Record In addition to Host (A) resource records, DNS servers also maintain other types of resource records that contain additional information about the hosts.
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Therefore, the full name for a computer in the DNS consists of two basic parts: a host name and a domain name. Note the similarity between the DNS name and an IP address, which also consists of two parts: a network identifier and a host identifier. The host name, as in the days before DNS, is a single word that identifies a specific com puter. Unlike host names in the early days, however, current host names do not have to be unique in the entire namespace; a host name only has to be unique in its domain.
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Understanding Domains
The domain name part of a DNS name is hierarchical, and consists of two or more words, separated by periods. The domain namespace takes the form of a tree that, much like a file system, has its root at the top. Just beneath the root is a series of toplevel domains, and beneath each top-level domain is a series of second-level domains.
Lesson 1
Determining Name Resolution Requirements
At minimum, the complete DNS name for a computer on the Internet consists of a host name, a second-level domain name, and a top-level domain name, written in that order and separated by periods. The complete DNS name for a particular computer is called its fully qualified domain name (FQDN).
Understanding FQDN Notation
Unlike an IP address, which places the network identifier first and follows it with the host, the notation for an FQDN places the host name first, followed by the domain name, with the top-level domain name last. For example, in the FQDN www.adatum.com, www is a host (or computer) in the adatum.com domain. In the adatum.com domain name, com is the top-level domain and adatum is the second-level domain. Technically, every FQDN should end with a period, repre senting the root of the DNS tree, as follows:
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